The catch composition and main fisheries-biological parameters of Juan Fernandez spiny rock lobster (Jasus frontalis), during the 1996-1997 fishing season were analyzed. A total of 22,997 specimens were measured at sea. Only 8.8% were over the minimum commercial size (98 mm). Cephalotoraxic lengths of males were comprised between 60.5 and 134.5 mm and between 60.5 and 137.5 in the females. Within the total catch sampled at sea, the females were in higher proportion (52.2%), and dominant in the range of 80.5 - 104.5 mm. Mean lengths varied between 83.9 and 88.7 in males, and between 83.7 and 89.9 mm in females. Before shipment to the continent, 4,048 commercial specimens were analyzed on land. Lengths of these specimens were between 93.5 and 152.5 mm in males and 94.5 and 146.5 in females. Sex proportion of the global landing was the same as the one recorded in the catch analyzed at sea, with a higher number of females over males (57.9%). Analyzing the length frequency distributions of landings and of catch at sea, seven molting stages were revealed, each one representing a year of age. The increment per molt is between 6.5 and 10.5 mm per molt, with the following growth curves: Lc = 212,5780* (1-exp-0,0707*(t-0)) en machos y, Lc = 214,4135* (1-exp-0,0697*(t-0)) en hembras Therelationship between the cephalotoraxic length (Lc, mm) and total weight (Wt, g) was defined by: W = 0,0017 * Lc2,7830 in males and Wt = 0,0034 * Lc2,6597 in females. Total mortality (Z), was estimated between 0.85 and 1.46 for males and between 0.85 and 1.91 for females. Accordingly with the used methods, the natural mortality rate (M) was defined between 0.07 and 1.24 for males and females, considering M = 0.18 to be the most likely value for both sexes